The hidden role of acetate in the PbSe nanocrystal synthesis

Article abstract of DOI:10.1021/ja061644v

Monodisperse spherical, star-shaped, and octahedral PbSe nanocrystals were synthesized via a hot injection method. We show that the shape and size of the colloidal PbSe nanocrystals are determined by the concentration of acetate and that only acetate-free reaction mixtures result in spherical nanocrystals. The presence of acetate leads to efficient oriented attachment of smaller PbSe nanoparticles along the 100 crystal axis. Comparing different synthesis procedures from the recent literature with our observations, we propose that the acetate, naturally present in insufficiently dried reaction mixtures, is responsible for many of the PbSe crystal shapes reported in the literature. In addition we show that it is possible to synthesize these star-shaped nanocrystals so monodisperse that they form ordered monolayers with crystal alignment. Copyright

Full text of DOI:10.1021/ja061644v

Published on Web 05/10/2006
The Hidden Role of Acetate in the PbSe Nanocrystal Synthesis
Arjan J. Houtepen,*,† Rolf Koole,*,† Danie¨l Vanmaekelbergh,† Johannes Meeldijk,‡ and
Stephen G. Hickey§,
Condensed Matter and Interfaces, UniVersity Utrecht, POB 80000, 3508 TA Utrecht, The Netherlands, Electron
Microscopy Utrecht, UniVersity Utrecht, POB 80.056, 3508 TB Utrecht, The Netherlands, and Institute for Physical
Chemistry, UniVersity of Hamburg, Grindelallee 117, D-20146 Hamburg, Germany
Received March 17, 2006; E-mail: a.j.houtepen@phys.uu.nl; r.koole@phys.uu.nl
In recent years interest in lead chalcogenide colloidal nanocrystals
(NCs) has grown considerably. Their rock-salt lattice leads to an
electronic band structure different from that of the typical II-VI
compounds with important consequences for the optical and
electrical properties in the regime of strong confinement. Further-
more, lead chalcogenide quantum dots hold promise for a wide
number of opto-electrical applications in the near infrared.1-3
Murray reported the first hot injection synthesis of monodisperse
PbSe quantum dots.2 In subsequent work, a myriad of geometries
of PbSe NCs were reported.4-7 Surprisingly, the dominant growth
mechanism of these geometries appeared to be the oriented
attachment of small PbSe NCs.4
We observed that it is crucial to use lead precursors completely
free of acetate in order to obtain spherical PbSe NCs. In this
communication, we present a systematic study of the role of acetate
in the growth mechanism and the resulting geometry of the NCs.
We show that the presence of even trace amounts of acetate in the
reaction mixture leads to starlike geometries formed by oriented
attachment. We propose that the presence of acetate is responsible
for many of the crystal shapes reported in the literature. In addition,
we present the first example of 2-D ordering and crystal alignment
of monodisperse, starlike NCs.
The preparation of the Pb-oleate precursor, used for the hot
injection synthesis,2 involves the following chemical reaction:
PbAc2‚3H2O + 2C17H33COOH a
Pb(C17H33COO)2 + 3H2O + 2HAc
Hence, if the Pb-oleate precursor is not completely dried, a
significant amount of water and acetate will be present in the
reaction mixture. Our first attempts to synthesize PbSe NCs resulted
in what appeared to be “stars of David”. The Pb-oleate was prepared
by heating the precursors to 120 °C for 30 min, possibly insufficient
for complete drying (see Supporting Information).
Figure 1. TEM images of PbSe crystal shapes grown at different
concentrations of acetate. (A-E) PbSe NCs grown under identical reaction
conditions with a Pb:Ac ratio as indicated in the images. Insets are a
diffraction pattern of a single octahedron (D) and a SEM image (E). (F)
PbSe NCs after 30 s of growth, in the initial stage of oriented attachment
toward octahedrons.
To obtain spherical NCs, we found that it is crucial to completely
dry the Pb-oleate precursor mixture by heating to >70 °C under
vacuum (<10-3 mbar) for more than 1 h; see Figure 1A. Therefore
we performed a systematic investigation on the effect of water and
acetate on the shape and size of PbSe NCs. This was done by
completely drying the precursor mixture and adding controlled
amounts of water and acetic acid.
The addition of a stoichiometric amount of water or an even
larger amount (Pb:H2O ) 1:15) to the carefully dried injection
mixture did not result in a deviation from the spherical shape (see
Figure S2 in the Supporting Information). In contrast, the addition
of acetic acid to the injection mixture resulted in a dramatic change
in crystal shape and size; see Figures 1B-E. From these images it
is apparent that the addition of acetic acid leads to the starlike crystal
shape; the amount of acetic acid has a strong effect on the size
(10-120 nm) of these crystals. The “diameter” of the NCs is shown
as a function of the Pb:Ac ratio in Table S1.
The hexagons in the TEM images of the largest crystals (Figure
1E) are actually 2D projections of octahedrons, as can be clearly
seen in a SEM image of the same NCs (inset in Figure 1E). The
transition from the small spherical particles to the large octahedral
particles involves an intermediate shape. The growth in the 100
direction is faster, resulting in extensions in this direction. This
leads to star-shaped particles that show up as stars of David in the
TEM images (Figures 1B,C and S3A). For very large crystals the
extensions in the 100 direction become so large that all the {100}
† Condensed Matter and Interfaces, University Utrecht.
‡ Electron Microscopy Utrecht, University Utrecht.
§ University of Hamburg.
Present address: Institute of Physical Chemistry and Electrochemistry,
Technical University of Dresden, Bergstrasse 66b, 01062 Dresden, Germany.
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C O M M U N I C A T I O N S
facets have disappeared and perfect octahedrons result (Figures 1E
and S3B-E). The formation of octahedrons by increasing the 100 /
111 growth ratio has previously been observed for PbS NCs.8
Lu et al. proposed that these octahedral crystals are formed by
the aggregation of smaller PbSe clusters.5 Our observations confirm
this mechanism. Figure 1F shows a TEM image of PbSe crystals
that were removed from the reaction mixture 30 s after the injection.
A mixture of small crystallites of ∼4 nm in diameter and aggregates
of 2-6 of those crystallites is visible in the initial stage of growth
toward octahedrons. The final stars or octahedrons are single
crystalline, as is illustrated by the discrete spots in the diffraction
pattern of a single octahedron (inset in Figure 1D, Figure S4). These
observations strongly suggest that those smaller crystallites self-
assemble into the star-shaped PbSe crystals. This is essentially the
same mechanism of oriented attachment that was proposed by Cho
et al.4 for the formation of PbSe nanowires and nanorings. The
formation of octahedrons implies that the oriented attachment of
smaller PbSe crystals is fastest in the 100 direction, ultimately
resulting in the elimination of all {100} facets. This is in accordance
with observations of Cho et al.4 In the formation of many different
crystal shapes they observed only oriented attachment along the
100 axis. As a possible explanation for this, they showed that the
dipole moment of quasi-spherical particles is strongest along the
100 axis and that this dipole orientation is the most frequent.4
The formation of octahedrons enhances this effect: the dipole
increases with the area of the polar {111} surface.
Figure 2. Self-assembled layers of star-shaped PbSe nanocrystals, viewed
along the 111 axis (A) and the 110 axis (B).
oriented attachment and many of the resulting crystal shapes
reported in the literature.4,5,11
In the presence of small amounts of acetate (Pb:Ac ≈ 5:1) the
star-shaped NCs can be made very monodisperse. The narrow size
distribution results in a striking long-range order in the 2-D packing
of star-shaped NCs (see Figure 2) and crystal alignment at the {100}
faces (see also Figure S7). The reason for this alignment may again
be the dipole moment along the 100 axis.
The formation of self-assembled structures of anisotropic NCs
with crystal alignment offers possibilities for the engineering of
functional NC solids. The expected anisotropy in the quantum
mechanical coupling may lead to new classes of electronic materials.
The optical and electrical properties of such a self-assembled
structure may strongly depend on the direction that is probed.
In conclusion, we have shown that acetate, naturally present in
insufficiently dried reaction mixtures, is a key factor in the growth
mechanism and the final PbSe NC shape and size.
The addition of acetic acid to the reaction mixture leads to a
(partial) replacement of oleate at the Pb sites on the NC surface.
Because acetate is so much smaller than oleate, this strongly reduces
steric hindrance between different crystals, which in turn drastically
increases the rate of oriented attachment. This explains why an
increasing concentration of acetate leads to larger NCs. The final
size and shape of the crystals do not depend strongly on the growth
temperature between 110 and 230 °C. The acetate concentration is
by far the most important parameter for this.
Acknowledgment. Financial support from the European Union
network “FULLSPECTRUM” (SES6-CT-2003-502620) is grate-
fully acknowledged.
Lu et al. make no mention of the reason their synthesis results
in star-shaped crystals. They prepare their Pb-oleate precursors by
heating to 150 °C for 30 min under an argon stream.5 We have
used this exact drying procedure and found that it results in a
mixture of quasi-spherical and octahedral NCs (see Figure S5). In
contrast, Wehrenberg et al. and the Murray group report that they
dry their precursors at >85 °C under Vacuum for at least 1 h3,9,10
when they prepare quasi-spherical PbSe NCs. Interestingly the
Murray group reports the formation of octahedral-shaped PbSe NCs
when they do not dry under vacuum. This synthesis was performed
in the presence of primary amines, and they speculate that the
octahedral shape is caused by blocking of the {111} facets by these
amines.4 We have repeated this experiment, but with a reaction
mixture that was carefully dried under vacuum, and we have
observed only quasi-spherical NCs (see Figure S6). It is important
to note that adding 4.8 mmol of hexadecylamine (HDA) does not
result in an apparent change in NC size or shape, whereas the
addition of 0.17 mmol of acetic acid has a marked effect on both
(see Figure 1B). On the basis of the observations reported here we
propose that heating the precursor mixture to 150 °C for 30 min
under an argon stream leads to incomplete drying and the inherent
presence of acetate at the surface of the synthesized NCs. We
propose that this acetate is responsible for the high degree of
Supporting Information Available: Experimental details and TEM
images of different syntheses. This material is available free of charge
via the Internet at http://pubs.acs.org.
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